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Moravian College • Preservation Master Plan can permanently damage masonry. Acidic cleaners or highly alkaline cleaners can damage historic materials and should be avoided. Prior to cleaning, the proposed methods should be tested in an inconspicuous area to evaluate potential adverse impacts. Cleaning solutions should start with the lowest recommended concentration and gradually be increased to find the appropriate level. Although it is time-consuming, the best way to determine the long-term effects of a cleaning process is to treat a small portion of the wall and observe it over a seasonal cycle. Staff and contractors applying the treatment follow the manufacturer’s instructions exactly. If chemicals are improperly applied, they can result in permanent damage that far outweighs any benefit of cleaning. Abrasive sandblasting should never be used on historic masonry because it is irreversible and extremely damaging. Sandblasting accelerates deterioration of historic masonry materials and has a profound negative impact on a building’s historic character. Sandblasting removes the hard, protective surface of the masonry, especially brick, and breaks mortar joints, leading to moisture penetration. If masonry surfaces were painted historically, they should remain painted. Paint has a specific protective function and often plays a part in the overall historic design and appearance. Paint clings tenaciously to the tiny pores and fissures of a masonry wall and its mortar joints. For this reason, paint on masonry is difficult to remove and should not be attempted only for aesthetic reasons. If conditions warrant paint removal, it should be approached cautiously using the gentlest means possible. Test patches in inconspicuous locations should precede any large-scale or overall cleaning. Masonry Coatings Waterproof coatings, paint, or stucco should never be applied to uncoated masonry buildings to solve moisture-related problems or as a substitute for repointing and preventative maintenance. Over time, these band-aid repairs trap moisture within the walls and cause a host of related problems. In some cases, a breathable masonry coating may be desired, to conceal concrete or stucco repairs, or unsightly staining. It should be understood that most masonry coatings are difficult, if not impossible, to remove. Coating masonry must not be undertaken without considering the long-term effects. If a masonry coating is required, the selection of a true breathable coating is critical for the long-term performance of both masonry and coating. Opaque, breathable masonry coatings are currently available as acrylic latex paints specifically formulated for masonry. They reduce the breathability of the masonry by ten percent. In theory, it may be possible to remove these coatings in the future, but this has not yet been tested. Silicate mineral paints are also available for painting silicate-based masonry, such as brick and sandstone. Silicate mineral paints reduce the breathability of masonry by only three percent. However, a silicate mineral paint will permanently bind to the silicatebased masonry substrate if properly prepared and should only be used where permanent coatings are desired. A mockup should be prepared to evaluate the coating in situ prior to large scale treatment. Buildings that have been painted may prove to be a lasting maintenance problem. New coatings must be compatible with the existing paint to adhere properly. All loose or deteriorated paint should be removed and a new, breathable masonry coating, compatible with the existing paint substrate, should be applied. However, over time, paint will build up on the surface of the masonry, which may cause moisture to be trapped in the wall or cause the paint to fail. Sealants Waterproof building sealants should only be applied to joints in horizontal surfaces at coping units, sills, steps, and at projecting cornice, belt, and water table courses. These joints are particularly prone to water penetration. Proper sealant installation involves installation of a foam backer rod with the correct diameter for the size of the joint. Sealant must be installed against the backer rod, forming a concave joint between the masonry units. Flush sealant joints that do not have a concave shape have been improperly installed. Do not allow sealants to overlap the outside face of the masonry, as it will cause the sealants to fail prematurely. Do not use sealant in joints on vertical wall surfaces because it will trap moisture within the wall and lead to deterioration. Masonry Repointing Repointing is the repair of deteriorated mortar joints, accomplished by removing and replacing old, deteriorated mortar. Appropriate repointing, using preservation techniques, is critical to a historic John Milner Associates • October 2009 • Chapter 9 • Building Guidelines • 290
Moravian College • Preservation Master Plan masonry building’s physical condition and plays a significant role in integrity and appearance. When mortar joints fail in masonry, they allow water to flow into the areas around the masonry, creating a chain of events that can weaken the entire wall. Repointing techniques and mortar mixes will vary according to the type of masonry. Therefore, consultation with preservation contractors and professionals is strongly recommended before undertaking repointing projects. Masons trained in new construction usually lack expertise with historic masonry. Replication Mortar Mixes The mortar mix is critical to masonry function and aesthetics and will vary according to the type of masonry. Ideally the composition of the new mortar would duplicate that of the original mortar mix. However, it should be noted that even current mortar analysis techniques cannot accurately determine the actual original mix; there are far too many variables. Mortar analysis techniques can provide subjective data on properties of the mortar such as probable binder content, hardness, air content, and color. However, no technique available today can accurately determine the actual original mix. An historic mortar analysis can, however, provide valuable information for characterizing the original mortar. This simple acid digestion and gravimetric analysis is usually performed by a building conservator for a modest cost. It is highly recommended as the first step in matching existing mortars for spot repointing. The analysis releases the original sand, allowing analysis of sand color, mineralogical content, and grain size; matching the original sand is important to finding a good replication mix. Iron oxide pigments may be added to match the color of historically pigmented mortars. Multiple test panels are usually needed to achieve the right color and texture match. In addition to mortar analysis, it is critical to understand the strength and permeability of the existing historic masonry. Knowledge of the physical properties of the existing masonry will aid in choosing a mortar mix with compatible characteristics. The new mortar must be weaker and more permeable than the existing masonry, while achieving good bond with the masonry units. It is important to evaluate the performance characteristics of the mortar mix as well as its appearance. Mortar installed prior to 1920 was typically composed of lime, which is more flexible and permeable than Portland cement-based mortars. Today’s high-strength, commercially-available Portland cement mortars are inappropriate for historic masonry buildings because they are hard and dense. Modern mortars are designed to match the characteristics of new, unweathered stone and contemporary hard brick, not historic masonry. Commercially-available masonry mortars should also be avoided because they can contain unnecessary, and undisclosed, additives and fillers. In addition to these functional concerns, modern mortars have a brighter and more uniform appearance than historic mortars. When hard Portland cement-based mortars are used for pointing historic masonry, the mortar is often harder than the masonry. Adding mortar that is too hard or dense to the soft or weathered historic masonry leads to accelerated masonry deterioration. The hard mortar traps moisture within the wall, which then cannot escape through the joints as it would normally. Instead, the moisture enters the softer and more permeable masonry and causes spalling, cracking, and deterioration. In historic masonry buildings, soft mortar joints are flexible enough to allow for the seasonal expansion and contraction of the wall. Modern buildings control this movement through expansion joints. If a historic masonry wall is pointed with a harder, less-flexible mortar, the masonry becomes the weakest link. The masonry, not the mortar joints, will crack, spall, or deteriorate as it absorbs the stress caused by seasonal changes and movement. If left unchecked, this masonry deterioration can result in structural failure of the wall. Non-hydraulic lime is a traditional building material produced by burning limestone and other calcium carbonate-rich materials, like shells. The resulting burned material is then slaked, or combined with water, to create a plastic, highly workable lime putty. The lime putty is mixed with sand to create the final mortar. Alternatively, dry, unslaked lime was “hot mixed” with sand and water to form the mortar. Non-hydraulic lime putty is commercially available. However, proper installation and curing is absolutely essential. Non-hydraulic lime mortars are extremely susceptible to shrinkage and failure from freezing temperatures. Currently, the majority of masons are comfortable with using combined Portland cement and hydraulic John Milner Associates • October 2009 • Chapter 9 • Building Guidelines • 291
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